Semiconductor devices such as a dynamic and nonvolatile random access memory ("DRAM") have decreased in size and increased in capacity dramatically over the last 20 years. As the capacity of memory cells has increased and the size has decreased, the design of the cells has become increasingly complex in order to preserve sufficient electrical capacitance to hold the electrical charge representing the stored data.
In the past silicon dioxide has been used as the dielectric in the capacitors of DRAM cells. However, silicon dioxide has a relatively low dielectric constant of about four. There has recently been experimentation with the use of other dielectric materials in order to increase the electrical capacitance in very small and complex cells. Some of this work has focused on the use of ferroelectric materials such as lead zirconate titanate ("PZT") as the dielectric in the capacitor for the production of a ferroelectric random access memory ("FRAM").
Ferroelectrics are dielectric in nature and exhibit spontaneous polarization which can be reversed by application of a suitable electric field. The polarization in these materials responds to an external electric field in a hysteresis fashion and thereby the materials exhibit bi-stable properties (two distinct states of polarization) which remain even after removal of the electric field. It is this hysteresis feature that makes ferroelectrics suitable for nonvolatile memory storage. The dielectric nature of ferroelectrics and their ability to display bistable properties can be used to make a ferroelectric capacitor which stores binary digital information based on the polarization state of the material. This opens up the possibilities of integrating a ferroelectric capacitor into the existing silicon and gallium arsenide VLSI technology to make a commercial nonvolatile random access memory. An example of a PZT device is described in U.S. Pat. No. 5,109,357 by Eaton.
Several problems need to be overcome before a commercially viable memory product is available. One of the foremost among these problems is the degradation properties of PZT ferroelectric devices. Degradation properties include fatigue, low voltage breakdown, and aging. A common source for these degradation properties is the interaction between defects in the materials and the ferroelectric-electrode interface/grain boundaries in the ferroelectric capacitor.
Considering the problem of fatigue, ferroelectrics are noted to lose some of their polarization as the polarization is reversed. This is known as fatigue degradation, and is one of the prime obstacles to forming high quality ferroelectric films. Fatigue occurs because of defect entrapment at the ferroelectric-electrode interface. Asymmetric electrode-ferroelectric interfaces and/or non-uniform domain distribution in the bulk can lead to asymmetric polarization on alternating polarity. This results in an internal field difference which can cause effective one-directional movement of defects like vacancies and mobile impurity ions. Since the electrode-ferroelectric interface is chemically unstable, it provides sites of lower potential energy to these defects relative to the bulk ferroelectric thereby causing defect entrapment at these interfaces (see Yoo, et al., "Fatigue Modeling of Lead Zirconate Titanate Thin Films", Jour. Material Sci. and Engineering). This entrapment will result in a loss in polarization in the ferroelectric.
To overcome this problem caused by defects, it is necessary to control the defect concentration, defect migration to the interface, defect entrapment at the interface, and the state of the interface itself. Lattice mismatch, poor adhesion, and large work function differences between the electrode and the ferroelectric causes the interface to be chemically unstable. Therefore, it is necessary to choose an appropriate electrode which can reduce the lattice mismatch, work function differences, and the adhesion problem at the interface. The existing, commonly-used, metal electrodes such as Pt, Au, etc., do not satisfy these criteria because of the large differences in crystal structures between the electrode (metal) and the ferroelectric (ceramic), and because of the work function differences. To control the defect migration and entrapment it is necessary to reduce the abrupt compositional gradient between the electrode and the ferroelectric.
Another obstacle to integrating PZT films into the existing semiconductor process is the high temperature post-deposition annealing needed to form the desirable perovskite phase. This annealing is required because most of the as-deposited films are amorphous, and they form an intermediate nonferroelectric pyrochlore phase before the formation of the perovskite phase. The transformation temperatures for initial perovskite formation and complete perovskite formation of the PZT films are functions of the compositions and the type of substrate used for deposition. Typical annealing temperatures for 53/47 films vary from 650.degree. C. to 750.degree. C. At these annealing temperatures, interdiffusion among the PZT films, the contact electrodes, and the underlying metallization becomes a concern; furthermore, thermal stress developed during the high temperature annealing may affect the long-term reliability of the device.
A variety of techniques have been used for the deposition of ferroelectric thin films. In general, the thin film deposition techniques can be divided into two major categories: (1) physical vapor deposition (PVD) and (2) chemical processes. Among the PVD techniques, the most common methods used for the deposition of ferroelectric thin films are electron beam evaporation, rf diode sputtering, rf magnetron sputtering, dc magnetron sputtering, ion beam sputtering, molecular beam epitaxy (MBE), and laser albation. The chemical processes can be further divided into two subgroups, i.e., metalorganic chemical vapor deposition (MOCVD), and wet chemical processes including sol-gel process and metalorganic composition (MOD). The third, newer process also used in the deposition of ferroelectric thin films is the liquid source delivery method.
The PVD techniques require a high vacuum, usually better than 10.sup.-5 torr, in order to obtain a sufficient flux of atoms or ions capable of depositing onto a substrate. The advantages of the PVD techniques are (1) dry processing, (2) high purity and cleanliness, and (3) compatibility with semiconductor integrated circuit processing. However, these are offset by disadvantages such as (1) low throughput, (2) low deposition rate, (3) difficult stoichiometry control, (4) high temperature post-deposition annealing, and (5) high equipment costs.
The advantages of the wet chemical process are (1) molecular homogeneity, (2) high deposition rate and high throughput, (3) excellent composition control, (4) easy introduction of dopants, and (5) low capital cost, since deposition can be done in ambient condition and so no vacuum processing is needed. The major problems due to this wet process are (1) film cracking during the post-annealing process and (2) possible contamination which results in difficulty in incorporating this technique into semiconductor processing. However, because it provides a fast and easy way to produce complex oxide thin films, this wet chemical process has a very important role in the investigation of the interrelationship among the processing, the microstructure, and the property of the films.
Of all the above mentioned techniques, the metalorganic chemical vapor deposition (MOCVD) technique appears to be one of the most promising because it offers advantages of simplified apparatus, excellent film uniformity, composition control, high film densities, high deposition rates, excellent step coverage, and amenability to large scale processing. The excellent film step coverage that can be obtained by MOCVD cannot be equaled by any other technique. Purity, controllability, and precision that have been demonstrated by MOCVD are competitive with the MBE technique. More importantly, novel structures can be grown easily and precisely. MOCVD is capable of producing materials for an entire class of devices which utilize either ultra-thin layers or atomically sharp interfaces. In addition, different compositions can be fabricated using the same sources.
The first successful deposition of oxide-based ferroelectric thin films by CVD was reported by Nakagawa, et al. in "Preparation of PbTiO.sub.3 ferroelectric thin film by chemical vapor deposition", Jpn. J. Appl. Phys., 21(1), L655 (1982). They deposited PbTiO.sub.3 films on Pt-coated silicon wafers by using TiCl.sub.4, PbCl.sub.2, O.sub.2 and H.sub.2 O as source materials. Several problems arose from their studies: (1) very high evaporation temperature (700.degree. C.) was required of PbCl.sub.2, (2) poor ferroelectric properties (P.sub.r =0.16 .mu.C/cm.sup.2 and E.sub.c =14.5 kV/cm), (3) poor composition uniformity in the bulk of PbTiO.sub.3 films, and (4) crystallographic imperfections due to water and/or chloride contamination. Obviously, chlorides are not ideal precursors for the CVD process. In general, metalorganic precursors have relatively high vapor pressures at lower temperatures. By carefully selecting the organic compounds, the undesirable contaminations in the films can be completely excluded. Metalorganic compounds are now used almost exclusively for the deposition of oxide thin films.